The present invention is directed to a process for recovery of lithium and other minor constituents from salt water bodies.
Valuable metals and salts are found in nature as dissolved ions in brine. Methods for recovery of minor constituents in such brines generally concentrate such constituents through evaporation of the brine. The concentration of the minor constituents can be effected either through chemical precipitation of the major constituents as described in U.S. Pat. No. 4,271,131 or by physical precipitation of the major constituents and separation of such precipitates from a brine which is rich in the minor constituents.
Recovery of commercially valuable compounds such as table salt (halite NaCl), potassium fertilizer (sylvite KCl) and the like from natural brines from the ocean, salt lakes, or underground by fractional crystallization where salts of successively lower solubility are precipitated in successive ponds is known. Chloride brines obtained from the Qaidam Basin of northwest China or from the Dead Sea for example, contain mainly sodium, potassium and magnesium ions. Fractionation by solar evaporation is carried out to precipitate halite, carnallite and bischoffite in successive ponds.
The oldest commercial process for recovery of lithium is by mining minerals such as those found in pegmatite, and then extracting lithium from the rocks. The process is very expensive and most of such mines are no longer operating.
The current processes for extraction of lithium from brines in California and in Chile employ brine evaporation and chemical fractionation to obtain a lithium concentrate. The lithium concentrate is processed in a refinery to produce a pure lithium compound such as lithium chloride or lithium carbonate.
The process utilizes chemical fractional crystallization, and has been used in California and Chile because the concentrations of the major ions in the brines, such as Mg.sup.++, K.sup.+, and Na.sup.+ are not high. The brines of the Clayton Valley, Calif. have a chemical composition typified by the following:
mg/l Ca Mg Na K Li C1 SO.sub.4 HCO.sub.3 420 190 6500 400 23 11000 460 930
In recovering lithium from the Clayton Valley underground brines are pumped into evaporating ponds, where calcium carbonate and calcium sulfate are almost totally precipitated and sodium chloride, potassium chloride and magnesium chloride are partially precipitated through chemical reaction with reagents. The residual effluent is a lithium concentrate which has a large enough volume to be separated from the precipitates in the harvest pond. The final product at Clayton Valley, after processing, is pure lithium carbonate.
In arid regions such as the Qaidam Basin of China or the Dead Sea, the brines are primarily chloride brines. After the exploitation of potash salts through fractional crystallization, the waste brine is almost a pure solution of magnesium chloride containing a minor amount of lithium, boron, bromine, iodine, and other minor constituents.
Minor constituents in these brines are present in such low concentrations that they cannot be separated from the major constituent, magnesium chloride by a single fractionation step. The Mg/Li ratio in Qaidam Basin or Dead Sea brines is typically in the order of 1000 to 1. To economically recover a trace element such as lithium from such brines, the ions of the desired element must reach a sufficiently high concentration. To obtain such high concentration, a substantial volume of brine is evaporated almost to dryness. The volume of the residual liquid which is enriched is too low to be separated from the salt mass precipitated from a simple progressive evaporation. Consequently, minor constituents are commonly trapped in the crystal matrix of other salts or occur as impurities in such salts. Heretofore, lithium was recovered by removal of magnesium from brines by chemical reaction with other salts, such as calcium oxide. This method is not economical if the amount of magnesium to be removed is too large because of the cost involved. Since minor elements such as lithium could not be practically recovered, and the major residual element, magnesium, could not be economically recovered, the residual brines left over from potassium recovery at sites such as the Qaidam Basin were commonly discarded as waste. Residual brines from the Dead Sea were utilized for recovery of magnesium and bromine, but not lithium.
Theoretically, minor salts, such as lithium, bromine, iodine, boron and the like will finally precipitate after the precipitation of major salts when this concentration also exceeds the equilibrium value. In a brine containing magnesium, sodium, and potassium chloride as major constituents, the ionic concentration of those constituents is hundreds or thousands times more than the minor constituents such as lithium, bromine, iodine, boron and the like. When the brine is evaporated, the major constituents in the brine will first be precipitated as one or more salts. From chloride brines, bromine and iodine could be occluded in the chloride crystals as a solid-solution. Some minor constituents such as lithium will, however, remain in solution until the concentration is sufficiently high to precipitate out, e.g., as lithium chloride. Experiments have shown that lithium will not precipitate until the ratio of major ions, such as magnesium, to lithium in chloride solutions, will be close to unity. Lithium will thus remain in solution and will only precipitate from solution when it is highly concentrated.
Until now, a lithium concentrate could not be obtained by current methods from brines such as those from the Dead Sea or from the Qaidam Basin because those brines have excessively high concentrations of major ions, particularly magnesium chloride.
The composition of typical brines from Charhan Lake in the Qaidam Basin, for example, in contrast to that of Clayton Valley brine from which recovery of lithium is possible with the existing methods, as expressed in milligrams per liter is:
 Ca Mg Na K Li Cl SO.sub.4 HCO.sub.3 Clayton Valley 420 190 6500 400 23 11000 460 930 Qaidam (orig.) minor 57890 29050 14910 13 228590 minor minor Qaidam (residue minor 122010 1220 293 133 359470 minor minor after Carnallite ppt.)
The precipitation of major salts will cause enrichment of some soluble minor constituents. When a brine saturated with a major ionic component, e.g. magnesium, is evaporated to half of its water volume, supersaturation causes half of that major ionic component to be precipitated. Consequently the major ionic concentration remains about the same at the saturation value. At the same time, a soluble minor constituent such as lithium would remain unsaturated despite the water loss, its concentration in this second stage of residual solution thus being doubled. When that brine still has a large enough volume in a liquid/solid mixture, it can be removed from the precipitated salt by physical separation, e.g. draining or pumping, and introduced into another evaporating pond, to be evaporated to half of its water volume again. The major ion concentration still remains about the same at its saturation value, and the concentration of a soluble minor constituent would, in a third stage of evaporation, be quadrupled. If this stepwise fractionation by a factor of 2 is repeated 10 times, the concentration of a soluble minor constituent such as lithium could be increased 2.sup.10 or 1024 times. If a stepwise fractionation by a factor of 3 is repeated 5 times, the concentration of a soluble minor constituent could be increased 3.sup.5 or 243 times. The concentrate of such a minor constituent could then be refined by chemical processes in in-situ reactors such as described in co-pending application, Ser. No. 08/403,364, filed Mar. 14, 1995, or in a refinery.
Using the current commercial practices to extract lithium chloride concentrate from brines, the amount of magnesium chloride precipitated from Qaidam Basin brines is 30 to 60 times greater than that precipitated from California brines. The residual solution enriched in lithium from Qaidam brines thus has such a small volume compared to the bulk of the precipitate that the enriched lithium brine cannot be easily separated from the salt precipitates. If the brine is evaporated to dryness, the lithium becomes an impurity at a magnesium chloride concentration of about 0.1% or less. It is not economical to extract lithium from a mixture of such low concentration. Therefore, neither the lake brines, nor the brines remaining after potash recovery from Qaidam can be used for lithium recovery. The brines remaining after potash recovery from Dead Sea brines are presently processed to recover magnesium and bromine, but not lithium.
To make one liter of lithium chloride brine-concentrate that has a magnesium/lithium ratio of 10 or less and contains 10 grams of lithium from Qaidam brines, the amount of magnesium chloride that has to be precipitated from that volume of Qaidam brine is more than 15,000 grams. If all the waste magnesium is precipitated in a single step, the residual solution enriched in lithium would have a very small volume compared to the bulk of the precipitate so that the lithium rich brine would remain in the pore space of magnesium chloride precipitates or it would be occluded by the crystalline material as a liquid inclusion.
Heretofore, lithium was recovered by removal of magnesium from the brines by chemical reaction with sodium carbonate or by physically evaporating the brine in stages. The chemical reaction method is not economical if the amount of magnesium to be removed is too large because of the cost of using a reagent such as sodium carbonate. Although removal of magnesium chloride in successive physical separation stages such as described in our co-pending application Ser. No. 08/503,587, provides a process superior to those of the prior art, since physical instead of chemical separation is used, the process may be more costly because it requires the use of a multiplicity of evaporative stages.
The process described in our co-pending application provides the design of an evaporation system in which the precipitate of the major constituent such as magnesium chloride is removed in successive physical separation stages. By removing the precipitate in a series of evaporating ponds, the final brine-precipitate ratio in the recovery pond is large enough for the brine enriched in the minor constituent to be separated by drainage or by pumping.
In our co-pending patent application, we described three alternative arrangements of ponds: (1) using a very large number of ponds initially and harvesting the brine enriched in the minor constituent in the last pond, (2) using very large ponds initially and harvesting the brine enriched in the minor constituent in the last, smaller pond, and (3) using a series of successive ponds of the same size containing brines with higher and higher concentration of the minor constituent to be recovered, and harvesting the brine, greatly enriched in the minor constituent, in the last pond.
It is an object of the present invention to provide for a more economic means than was heretofore available for enrichment and recovery of minor constituents in brine.
It is a further object of the present invention to improve the rate of evaporation of brine under normal conditions to provide a more economical process for recovery of lithium and other minor brine constituents.